1. Field of the Invention
The present invention relates to a circuit for controlling a fluorescent signaling device, in which the circuit contains an alternating current (a.c.) supplied filament, several grids assigned to the various signal points, signaling elements consisting of segments, direct current (d.c.) voltage source which generates a voltage between the filament as cathode and the signaling elements as anode, and a driver circuit whose gating outputs are connected to the individual grids of the fluorescent signaling device.
2. Description of the Prior Art
A conventional circuit of the prior art is shown in FIG. 1. The fluorescent signaling device consists of elongated, evacuated vitreous body 1 within which filament 2 is provided and current is supplied from a.c. voltage source 5. Filament 2 faces signaling elements 3a-3h which are constructed as 7-segment signaling elements, of which FIG. 2 shows only one segment in each case. Furthermore, grid 4a-4h is assigned to each point of the fluorescent signaling device. The negative terminal of the d.c. voltage source for supplying the fluorescent signaling device is connected to center tap 6 of heating voltage source 5 and imparts a negative bias to filament 2 which acts as a cathode. The positive terminal of direct voltage source 7 is connected, via driver circuit 9, to signaling elements 3a-3h which at the instant concerned are to be stimulated and lighted. Driver circuit 9 controls via gating outputs 8 individual grids 4a-4h and thus determines the point that is to show the signal. D.C. voltage source 7 supplies about 50 volts, while the heating a.c. voltage supplies about 5 volts.
A.C. voltage U.sub.H for supplying filament 2 combines with voltage U.sub.V of voltage source 7, so that cathode/anode voltage U.sub.AK at the center points of the signaling device is constant while at the outer points of the signaling device it varies in a 50 Hz rhythm. In FIG. 2 the voltage variation of the assigned heating filament area for signaling element 3d, ranges between 0 and 1/4 of heating voltage U.sub.H, for an average of 1/8 of the heating voltage. For the signaling element 3a, however, the voltage variation of the corresponding filament area is between 3/4 and 4/4 of the heating voltage, for an average of 7/8 of the heating voltage.
To avoid these variations in the cathode/anode voltage which lead to visible variations in the brightness of the signal, it is customary to select a multiplex frequency signal that is larger than 100 Hz, because then the brightness variations are so rapid that the human eye can no longer perceive or detect them.
However, when the driver circuit in the microprocessor for the signaling device is integrated, the multiplex frequency must be in the range of 50 Hz; and therefore visible variations in brightness occur because of the frequency difference between the 50 Hz filament voltage and the multiplex frequency (surge).
In order to avoid these brightness variations, it is feasible to change the frequency of the filament voltage, either to 0 Hz (d.c. supply) or to about 20 kHz. The d.c. filament however, leads to a drop of the cathode/anode voltage from one end of the signaling device to the other end, so that normally a corresponding drop in brightness results. This drop in brightness could be structurally compensated for, by a modified filament. This, however, leads to cost increases, which, particularly in the case of smaller quantities, is substantial. An increase in the frequency of the heating a.c. voltage by the use of blocking oscillators, requires a high circuit expense for the ferrite transmitter, the transistors for controlling the relatively large currents, and for condensers.
The object of the present invention is to provide a simple, economical circuit which, in spite of the 50 Hz feeding of the filament and a multiplex frequency in the range of 50 Hz prevents visible variations in signal brightness.